1. Field of the Invention
This invention relates to current limiting devices for general circuit protection including electrical distribution and motor control applications. In particular, the invention relates to current limiting devices that are capable of limiting the current in a circuit when a short-circuit or high current condition occurs.
2. Description of Related Art
There are numerous devices that are capable of limiting the current in a circuit when a high current condition occurs. One known limiting device includes a filled polymer material that exhibits what is commonly referred to as a PTCR (positive-temperature coefficient of resistance) or PTC effect. U.S. Pat. No. 5,382,938, U.S. Pat. No. 5,313,184, and European Published Patent Application No. 0,640,995 A1 each describe electrical devices relying on PTC behavior. The unique attribute of the PTCR or PTC effect is that at a certain switch temperature the PTCR material undergoes a transformation from a basically conductive material to a basically resistive material. In some of these prior current limiting devices, the PTCR material (typically polyethylene loaded with carbon black) is placed between pressure contact electrodes.
U.S. patent application Ser. No. 08/514,076, filed Aug. 11, 1995, now U.S. Pat. No. 5,614,881, issued Mar. 25, 1997, the entire contents of which are herein incorporated by reference, discloses a current limiting device. This current limiting device relies on a composite material and an inhomogeneous distribution of resistance structure.
Current limiting devices are used in many applications to protect sensitive components in an electrical circuit from high fault currents. Applications range from low voltage and low current electrical circuits to high voltage and high current electrical distribution systems. An important requirement for many applications is a fast current limiting response to minimize the peak fault current that develops.
In operation, current limiting devices are placed in a circuit to be protected. Under normal circuit conditions, the current limiting device is in a highly conducting state. When a high current condition occurs, the PTCR material heats up through resistive heating until the temperature is above the "switch temperature." At this point, the PTCR material resistance changes to a high resistance state and the high current condition current is limited. When the high current condition is cleared, the current limiting device cools down over a time period, which may be long, to below the switch temperature and returns to the highly conducting state. In the highly conducting state, the current limiting device is again capable of switching to the high resistance state in response to future high current condition events.
Known current limiting devices comprise conductive composite material and standard flat electrodes that have generally planar surfaces, a low pyrolysis or vaporization temperature polymeric binder and an electrically conducting filler combined with an inhomogeneous distribution of resistance structure. The switching action of these current limiting devices occurs when joule heating of the electrically conducting filler in the relatively higher resistance part of the composite material causes sufficient heating to cause pyrolysis or vaporization of the binder.
During operation of known current limiting devices, at least one of material ablation and arcing occur at localized switching regions in the inhomogeneous distribution of resistance structure. The ablation and arcing can lead to at least one of high mechanical and thermal stresses on the conductive composite material. These high mechanical and thermal stresses often lead to the mechanical failure of the composite material. For a reliable operation, it is desirable to reduce high mechanical and thermal stresses.
Further, the ablation and arcing often produce a non-conducting, or at least a poorly conducting, residue from the expended filler. This residue, comprises various ablation products, and is located at an interface between the standard flat electrode and conductive composite material of the current limiting device after a switching event. This residue is undesirable since it is believed to increase the overall resistance of the current limiting device. The residue is distributed over the interface, so as to impair the overall operation of the current limiting device due to the added resistance. Since a current limiting device should have as low a resistance as possible, especially after a switching event, and have small an area as possible, the increased resistance, due to the residue, limits uses of current limiting devices with standard flat electrodes.